Semiconductor devices containing integrated circuits (ICs) or discrete devices are used in a wide variety of electronic apparatus. The IC devices (or chips, or discrete devices) comprise a miniaturized electronic circuit that has been manufactured in the surface of a substrate of semiconductor material. The circuits are composed of many overlapping layers, including layers containing dopants that can be diffused into the substrate (called diffusion layers) or ions that are implanted (implant layers) into the substrate. Other layers are conductors (polysilicon or metal layers) or connections between the conducting layers (via or contact layers). IC devices or discrete devices can be fabricated in a layer-by-layer process that uses a combination of many steps, including growing layers, imaging, deposition, etching, doping and cleaning. Silicon wafers are typically used as the substrate and photolithography is used to mark different areas of the substrate to be doped or to deposit and define polysilicon, insulators, or metal layers.
The localized oxidation of silicon (LOCOS) isolation method is used in many processes for manufacturing semiconductor integrated circuits. In LOCOS processing, active silicon areas on the surface of a silicon substrate (or epitaxial layer) can be electrically isolated by relatively thick insulating (or bulk) oxide regions. A patterned film of deposited silicon nitride can be used to selectively suppress oxide growth where an active silicon region is desired. Devices such as diodes, transistors, resistors, capacitors and other microelectronic structures are subsequently built in these active silicon regions between the insulating oxide regions.
As semiconductor device dimensions shrink in size and pitch, it becomes increasingly difficult to grow a thick and robust LOCOS oxide between closely spaced silicon regions. This is because the oxide layer thins as it approached the active silicon edge, forming the classic “bird's head” profile. Therefore, the full desired thickness may never be achieved if the active areas are so close that the opposing bird's heads intersect. Compounding this problem, after growth the field oxide is exposed to several subsequent processing steps that diminish its thickness, and further reduces its effectiveness as an ion implant blocking agent. These include the oxide etches associated with the oxide spacer formation and other processing steps. Their effect is shrinkage of all isolation oxide regions both laterally and vertically. The oxide between closely spaced active regions is affected proportionally more since it is thinner at the start.